1. Field of the Invention
The present invention relates to a leadframe material for a semiconductor and a process for producing the same. More particularly, the present invention is concerned with an iron-base or copper-base leadframe having a good etchability, an excellent fine fabricability and a high strength and a process for producing the same.
2. Description of the Related Art
Leadframe materials having various qualities and configurations are known as a leadframe material used for mounting of semiconductors. For example, it is a common practice to mold a leadframe base material rolled to a desired sheet thickness into a pattern having a predetermined configuration by etching or stamping and then subject the whole or predetermined positions of the base material to a surface treatment such as gold plating, silver plating, nickel plating or tin-nickel plating.
An iron-nickel-base Alloy 42 (42% Ni-Fe) , an iron-nickel-cobalt-base Kovar (29% Ni-13% Co-Fe) alloy and a copper-base alloy are used as the leadframe material, and the thickness is generally in the range of from 100 to 250 .mu.m.
In recent years, with the advance of high-density mounting of a semiconductor device, there is an ever-increasing tendency towards an increase in the number of pins and a reduction in the size of the pitch. In the case of mass production through the use of the above-described material, the limitation of the inner lead pitch in the etching is up to about 200 .mu.m. As the pitch becomes narrow and the number of the pins increases, it becomes difficult to ensure the inner lead flat width necessary for the wire bonding. Further, in this case, since the amount of the side etching increases, shortcircuiting is liable to occur between leads. In particular, the Alloy 42 which is mainly used in the high pin count leadframe is inferior to the copper alloy in the etchability, so that it is difficult to cope with an extreme increase in the number of pins and an extreme reduction in the size of the pitch.
In the conventional etching, it was necessary to reduce the flat width of a back side (bottom side) of the inner leads for the purpose of ensuring the flat width of a face side (top side). Care should be taken not to excessively reduce the flat width of the reverse face because it becomes difficult to maintain the flatness of the inner lead at the time of wire bonding process.
An increase in the number of pins and a reduction in the size of pitch have been attained through a reduction in the thickness of the leadframe material. In this method, however, the lack of the mechanical strength becomes significant with a reduction in the thickness of the leadframe materials. For this reason, high-strength materials of iron-base alloys or copper-base alloys have been developed. In this case as well, however, an extreme reduction in the thickness of the material is impossible from the viewpoint of the material strength, so that the lower limit of the thickness is considered to be in the range of from 80 to 125 .mu.m.
Further, the Alloy 42 and Kovar commonly used in the art as a leadframe material contain a considerable amount of Ni and Co, so that the unit price of the material is unfavorably high.